There are various systems of printing apparatuses which print information such as characters and images on sheet-like printing media such as print sheets and films. As a representative system of forming characters and images by attaching color materials on a printing medium, an ink-jet system using a printhead which ejects inks is known.
Photo printing which prints an image on glossy paper using an ink-jet printing apparatus to form a photo-quality image has prevailed. The ink-jet printing apparatus popularly uses a dye ink using a dye, which is easily dissolved in water, as a color material. A color material dissolved in a solvent in the dye ink easily infiltrates a fibrous interior of a printing medium. Therefore, after an image is printed, the surface shape of the printing medium is easily maintained, and gloss of the printing medium is maintained as that of the printed image. That is, when an image is printed on a printing medium which is excellent in gloss using a dye ink, an image which is excellent in gloss can be obtained. In other words, the ink-jet printing apparatus using the dye ink can easily gloss over an image due to improvement of gloss of the printing medium.
On the other hand, there are requirements for light resistance and water resistance for a printed matter. Dye molecules of a color material in the dye ink are degraded by light, and the color of a printed image is readily faded (low light resisitance). When a printed matter printed using the dye ink becomes wet with water, dye molecules infiltrated in the fibrous interior are dissolved in water, and an image unwantedly blurs (low water resistance). In order to solve problems about the light resistance and water resistance, the use of a pigment ink which uses a pigment as a color material has been increasing in recent years. Unlike the dye which exists in a solvent in a molecular state, the pigment exists in a solvent as particles as large as several ten nm to several μm. That is, color material particles of the pigment ink are large, and a printed matter with high light resistance can be obtained.
The pigment ink excels in light resistance and water resistance, but colorized specular reflection light of a printed image may often pose a problem in terms of image quality. An observer recognizes the specular reflection light as, for example, an image of an illumination reflected on the surface of the printed image. When the specular reflection light (and diffused light of an angle closer to the specular reflection light) is colorized, and an image of an illumination reflected on a printed matter is observed as a color different from the original illumination, such an image disturbs observation of an image, and is especially not preferable for a photo-printed image. As causes of coloring of the specular reflection light, a bronzing and thin-film interference are known.
The bronzing will be described below with reference to FIG. 1. Incident light 104 from a light source is reflected by a color material 102 printed on a printing medium 101. Specular reflection light 103 is light which is reflected in a direction of the same angle θ as an incident angle θ of the incident light 104. Since the reflectance of the color material 102 depends on the wavelength of light, a spectral distribution of the incident light 104 is different from that of the specular reflection light 103, thus recognizing the colorized specular reflection light 103. This is the bronzing, and the specular reflection light 103 is colorized with a color unique to the color material 102. Especially, it is known that the specular reflection light is colorized with magenta by a cyan color material.
The thin-film interference will be described below with reference to FIG. 2. Specular reflection light with respect to incident light 207 from a light source includes specular reflection light 204 reflected by the surface of a color material 203 printed on a printing medium 201 and specular reflection light 205 reflected by a boundary between the color material 203 and an underlying color material 202. Optical path lengths of the two specular reflection light rays 204 and 205 have a difference as large as a thickness of the color material 203, thus generating a phase difference between the two specular reflection light rays 204 and 205. As a result, the two specular reflection light rays 204 and 205 interfere with each other, thus coloring specular reflection light to be observed. This is the thin-film interference.
The bronzing and thin-film interference depend on a material near the surface of a printed matter, and their degrees of generation are also different depending on a structure near the surface. This structure is, for example, a ratio of an ink (covering ratio) which occupies the surface of a printing medium. That is, since the surface of a printed matter is configured by structures and materials which are different depending on colors or tones, colors of specular reflection light are different depending on the colors or tones. As a result, specular reflection light from an image configured by a plurality of colors is observed as different colors depending on image positions, thus providing a feeling of strangeness to an observer of the image.
As a method of solving this problem, a technique for overcoating a yellow ink on an image is available (Japanese Patent Laid-Open No. 2004-181688). Also, a technique for overcoating an ink (clear ink) containing a clear and colorless color material on the entire printed region of an image is available (Japanese Patent Laid-Open No. 2003-132350).
The method of Japanese Patent Laid-Open No. 2004-181688 mixes an ink which is not necessary for color reproduction (for example, the method adds a yellow ink upon reproduction of cyan). Mixing of the ink which is not necessary for color reproduction results in a decrease in saturation, and reduces a color reproduction range (color gamut).
Overcoating of a clear ink in the method of Japanese Patent Laid-Open No. 2003-132350 does not impose any influence on color reproduction, but it cannot always suppress the bronzing.